Hello - My company is adopting ITIL and they want to classify incidents (categorize issues).

I remember reading a few years ago that people were less likely to complete surveys over 5 questions long. I think these days a lot of surveys have been condensed even further - such as "Would you recommend? Y/N"

Anyway, I think the same principle may apply to forcing users to select issue categories/subcategories. A few categories is probably helpful but several categories and subcategories is probably a hindrance. I'm guessing that users are more likely to select Other or shortcut the process if they're presented with a complex selection metaphor with a lot of extraneous information.

Can anyone here direct me to any well-known usability principles or guidelines on this type of thing that would help me defend the use of a few categories instead of using several categories/subcategories?

You don't need your users to apply categories. Although it is useful if they do give a category from their perspective, it is not likely to be reliable in many cases. They can only categorize from symptoms and only the ones they see. What use would you hope to make of their categories? Support staff are far better placed to apply categories at all stages of the process. A lot depends on the kinds of users and the kinds of services._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718